U.S. patent number 5,942,052 [Application Number 08/708,916] was granted by the patent office on 1999-08-24 for surface treatment aqueous solution for metal.
This patent grant is currently assigned to Nippon Paint Co., Ltd.. Invention is credited to Masayuki Kamimura, Yasuo Matsuura, Satoshi Miyamoto.
United States Patent |
5,942,052 |
Kamimura , et al. |
August 24, 1999 |
Surface treatment aqueous solution for metal
Abstract
A surface treatment aqueous solution is provided which is
superior in corrosion resistance and coating adhesiveness as well
as in ability of inhibiting the production of precipitate. A tinned
DI can, which has been degreased, is conversion coating processed
in a treatment bath filled with a surface treatment aqueous
solution containing at least phosphate ions, organophosphonate
compound and tin ions with a pH value of 5 or less. After the
conversion coating, the DI can is washed with water and dried. A
preferable organophosphonate compound is one in which a phosphorus
atom constituting a phosphonate group is bonded to a carbon
atom.
Inventors: |
Kamimura; Masayuki (Ichikawa,
JP), Miyamoto; Satoshi (Yokohama, JP),
Matsuura; Yasuo (Osaka, JP) |
Assignee: |
Nippon Paint Co., Ltd. (Osaka,
JP)
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Family
ID: |
13756434 |
Appl.
No.: |
08/708,916 |
Filed: |
September 5, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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421263 |
Apr 13, 1995 |
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Foreign Application Priority Data
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Apr 20, 1994 [JP] |
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6-081795 |
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Current U.S.
Class: |
148/250; 148/254;
148/261; 148/273; 148/260; 148/259 |
Current CPC
Class: |
C23C
22/08 (20130101) |
Current International
Class: |
C23C
22/05 (20060101); C23C 22/08 (20060101); C23C
022/00 () |
Field of
Search: |
;148/250,254,259,260,261,273 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0084593 |
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Aug 1983 |
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EP |
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0312176 |
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Apr 1989 |
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EP |
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52-53739 |
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Oct 1975 |
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JP |
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0005879 |
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Jan 1982 |
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JP |
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215178 |
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Jan 1990 |
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JP |
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Other References
Primary Examiner: Simmons; David A.
Assistant Examiner: Koehler; Robert R.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas, PLLC
Parent Case Text
This is a Continuation of application Ser. No. 08/421,263, filed
Apr. 13, 1995, abandoned.
Claims
What is claimed is:
1. A surface treatment aqueous solution for metal, containing at
least phosphate ions, at least one organophosphonate compound
selected from aminotri (methylenephosphonic acid),
1-hydroxyethylidene-1, 1-disphoshonic acid, ethylenediaminetetra
(methylenephosphonic acid), diethylenetriaminepenta
(methylenephosphonic acid) and their salts, and tin ions, with a pH
value being 5 or less.
2. The surface treatment aqueous solution according to claim 1,
wherein said organophosphonate compound is a compound of at least
one selected from aminotri (methylenephosphonic acid),
1-hydroxyethylidene-1,1-diphosphonic acid, and their salts.
3. The surface treatment aqueous solution according to claim 1,
wherein a supply source of said phosphate ions is at least one
selected from orthophosphoric acid and orthophosphate.
4. The surface treatment aqueous solution according to claim 1,
wherein a supply source of said tin ions is at least one selected
from stannous chloride, stannic chloride, sodium stannate, and
stannous sulfate.
5. The surface treatment aqueous solution according claim 1,
wherein the mole ratio of tin ion to organophosphonate compound is
0.08 to 8.4.
6. The surface treatment aqueous solution according to claim 1,
wherein the mole ratio of tin ions to organophosphate compound is
0.17 to 2.8.
7. The surface treatment aqueous solution according to claim 1,
wherein the amount of said phosphate ion contained in the aqueous
solution is 3 to 10 g/l.
8. The surface treatment aqueous solution according to claim 1,
wherein the amount of said phosphate ions contained is 1 to 30 g/l,
the amount of said phosphate ions contained is 1 to 30 g/l the
amount of said organophosphonate is 0.05 mmol/l to 50 mmol/l, the
amount of said tin ions is 0.005 to 5 g/l, the mole ratio of tin
ions to organophosphate compound is 0.08 to 8.4, and the pH value
of the aqueous solution is 2 to 5.
9. The surface treatment aqueous solution according to claim 1,
wherein the amount of said organophosphonate contained in the
aqueous solution is 0.5 mmol/l to 5 mmol/l.
10. The surface treatment aqueous solution according to claim 1,
wherein the amount of said tin ions contained in the aqueous
solution is 0.05 to 0.5 g/l.
11. The surface treatment aqueous solution according to claim 1,
wherein the amount of said phosphate ion contained is 3 to 10 g/l,
the amount of said organophosphonate compound is 0.5 mmol/l to 5
mmol/l, the amount of said tin ions is 0.05 to 0.5 g/l, the mole
ratio of tin ions to organophsophonate compound is 0.17 to 2.8, and
the pH value of the aqueous solution is 2 to 5.
12. The surface treatment aqueous solution according to claim 1,
wherein the pH value of the aqueous solution is 2 to 3.5.
13. The surface treatment aqueous solution according to claim 8,
wherein the pH value of the aqueous solution is 2 to 3.5.
14. The surface treatment aqueous solution according to claim 11,
wherein the pH value of the aqueous solution is 2 to 3.5.
15. A surface treatment aqueous solution for metal, containing:
(a) at least one selected from orthophosphoric acid and
orthophosphate with a phosphate ion amount of 3 to 10 g/l;
(b) 0.5 mmol/l to 5 mmol/l of at least one organophosphanate
compound selected from aminotri (methylenephosphonic acid),
1-hydroxyethylidene-1-1-diphosphonic acid, and salt thereof;
and
(c) 0.05 to 0.5 g/l of tin ions;
wherein a mole ratio of tin ions to organophosphonate compound is
0.17 to 1, and the pH value of the aqueous solution is 2 to 5.
16. A process of forming a conversion coating film on a surface of
tinned iron, comprising steps of:
preparing a surface treatment aqueous solution containing at least
phosphate ions, organophosphonate compound, and tin ions, with a pH
value being 5 or less; and
applying the surface treatment aqueous solution onto a surface of
tinned iron which has been degreased and water washed.
17. A process of forming a conversion coating film on a surface of
tinned iron, comprising steps of:
preparing a surface treatment aqueous solution containing (a) at
least one selected from orthophosphoric acid and orthophosphate
with a phosphate ion amount of 3 to 10 g/l, (b) 0.5 mmol/l to 5
mmol/l of at least one organophosphonate compound selected from
aminotri (methylenephosphonic acid),
1-hydroxyethylidene-1,1-diphosphonic acid, and their salt, and (c)
0.05 to 0.5 g/l of tin ions, with a mole ratio of tin ions to
organophosphonate compound being 0.17 to 1, and a pH value being 2
to 5;
applying the surface treatment aqueous solution onto a surface of
tinned iron which has been degreased and water washed.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface treatment aqueous
solution for providing a conversion coating on a metal surface such
as iron or iron alloy, especially on the surface of tinned iron.
This invention also relates to a use of the surface treatment
aqueous solution and to a process of applying the surface treatment
aqueous solution to the surface of tinned iron.
2. Description of the Prior Art
Iron cans are broadly used for various canned products, as a
container for food or soft drinks. Such cans are fabricated by
shaping a thin iron plate into a desired size of can using a
forming die or a drawing & ironing technique (DI technique). In
either method, a shaped can is generally provided with an
anticorrosion coating because the material itself is inferior in
corrosion resistance. As an anticorrosion coating, a thin film of
epoxy resin is ordinarily formed on the can's surface. Resin
coating is not directly provided on the iron surface, but on the
treated surface. Such a surface treatment includes a conversion
coating which is preferable in view of corrosion resistance and
coating adhesiveness especially for tinned iron cans.
Japan Kokai (Laid Open) H2-15178, entitled "Conversion Coating
Agent and Conversion Coating Bath", discloses a conversion coating
on a tinned can, using an aqueous solution containing
orthophosphoric acid and/or orthophosphate with a PO.sub.4 amount 1
to 30 g/l, oxalic acid and/or oxalate with a C.sub.2 O.sub.4 amount
0.005 to 5.0 g/l, and 0.005 to 0.5 g/l of dihydric tin ion, with a
pH value 3 to 5. The oxalic acid is added to a treatment bath as an
etchant and as a chelating agent for tin ions for preventing
sludge.
U.S. Pat. No. 4,927,472 entitled "Conversion Coating Solution For
Treating Metal Surface" discloses an aqueous solution containing 1
to 50 g/l of phosphate ions, 0.2 to 20.0 g/l of oxyacid ions, 0.01
to 5.0 g/l of tin ions, and 0.01 to 5.0 g/l of condensed phosphate
ions, with a pH value 2 to 6. The condensed phosphate ions are
added as an etchant as well as a chelating agent for tin ions for
preventing sludge.
Japan Kokai (SHO)52-53739 entitled "Surface Treating Method for Tin
Plate" discloses a method for treating a surface of a tinned steel
plate, using an aqueous solution in which one or both of
phosphorous acid and hypophosphorous acid is/are added to
phosphoric acid and the amount of free acid is adjusted to 5.0 to
50.0 g/l. The phosphorous acid and hypophosphorous acid are added
as an etchant or a coating accelerator.
U.S. Pat. No. 4,980,076 entitled "Fluoride and Chromium Free Acid
Etchant Rinse For Aluminum" discloses an acid etchant rinse for
aluminum containing orthophosphoric acid in an amount to give a
stoichiometric equivalent of 3.0 to 50 g/l as PO.sub.4.sup.3-, an
aluminum ion sequestrant component in an amount of 0.01 to 10.0
g/l, 20 to 170 ppm of ferric ion, and 0.02 to 3.0 g/l of H.sub.2
O.sub.2 or NO.sub.2.sup.-. As an aluminum ion sequestrant,
organophosphonate is employed.
However, the conversion coating agent shown in H2-15178 has a
problem that the oxalic acid contained in the treatment bath is
slightly inferior in chelation ability to tin ions, and therefore
it can not sufficiently hold tin ions. As a result, sludge is
produced in the treatment bath, which will block a nozzle.
It is known that condensed phosphate is easily hydrolyzed in a
strong acidic aqueous solution to be a phosphorate with a poor
degree of polymerization. For this reason, the conversion coating
solution disclosed in U.S. Pat. No. 4,927,472 has a problem that
the condensed phosphate contained in the solution is quickly
hydrolyzed at pH value 2 to 5 suitable for conversion coating, and
chelation ability to tin ion is extremely reduced, which results in
precipitate of tin salt and production of sludge. It is not
economical to continuously supply condensed phosphate ions as
hydrolysis advances. Further, the oxyacid promotes elution of iron
ions which consume the chelating agent, resulting in insufficient
trapping of tin ions and production of sludge.
In Japan Kokai (SHO)52-53739, inorganic hypophosphorous acid, such
as sodium dihydrogenphosphate or ammonium dihydrogenphophate, is
added as an etchant or coating accelerator. However, inorganic
hypophosphorous acid is inferior in chelation ability, and
therefore, accumulation of tin ions produces sludge.
The solution shown in U.S. Pat. No. 4,980,076 is an acid etchant
rinse for treating aluminum or aluminum alloy, where ferric ion and
an oxidizer are indispensable. This solution differs from s surface
treatment aqueous solution of the present invention in structure,
purpose and effect.
SUMMARY OF THE INVENTION
This invention was conceived to overcome the above described
problem, and it is an object of the invention to provide a surface
treatment aqueous solution capable of providing a conversion
coating superior in corrosion resistance and coating
adhesiveness.
In order to achieve the above object, a surface treatment aqueous
solution in accordance with the invention contains at least
phosphate ions, organophosphonate compound and tin ions, with a pH
value being 5 or less.
The organophosphonate compound is one in which a phosphorus atom
consisting of a phosphonate group is bonded to a carbon atom.
In another aspect of the invention, a use of the surface treatment
aqueous solution and a process of forming a conversion coating by
applying the surface treatment aqueous solution to a surface of
tinned iron.
A supply source of the phosphate ions (first component) is, for
example, orthophosphoric acid or orthophosphate. The orthophosphate
may be one that includes at least one substituent of alkaline metal
(such as sodium, potassium, lithium, etc.) or ammonium.
Alternatively, the orthophosphate may be one obtained by
neutralizing orthophosphoric acid with caustic alkali or carbonate
alkali.
The orthophosphoric acid and orthophosphate may be used separately
or all together. Combination of at least two kinds of
orthophosphate is also available. The concentration of
orthophosphoric acid or orthophosphate in the surface treatment
aqueous solution is preferably such that the phosphate ion
concentration is 1 to 30 g/l, and is more preferably, such that the
phosphate ion concentration is 3 to 10 g/l. When the concentration
of orthophosphonic acid or orthophosphate in the aqueous solution
is such that the phosphate ion amount is less than 1 g/l,
conversion coating film is not only sufficiently formed, but also
inferior in corrosion resistance. On the other hand, when exceeding
30 g/l, etching of the metal surface is accelerated too much and
the appearance of the can is damaged, in addition to inferior
corrosion resistance and coating adhesiveness.
The preferred organophosphonate compound (second component) is one
in which a phosphonate group (--PO.sub.3 H.sub.2) represented below
is bonded to a carbon atom. ##STR1##
Alternatively, the organophosphonate compound may be one in which
at least one of the hydrogen atoms in the phosphonate group is
substituted with alkaline metal (such as sodium, potassium,
lithium, etc.) or ammonium, or may be one that is obtained by
neutralizing organic phosphonic acid by caustic alkali, carbonate
alkali, or ammonia.
More specifically, aminotri (methylenephosphonic acid) represented
by formula 1, 1-hydroxyethylidene-1,1-diphosphonic acid represented
by formula 3, ethylenediaminetetra (methylenephosphonic acid)
represented by formula 6, diethylenetriaminepenta
(methylenephosphonic acid) represented by formula 7, or their salt
(represented by formula 2, 4 and 5, respectively) are listed. Such
salts include at least one substituent for each phosphonate group,
the substituent being preferably alkaline metal (sodium, potassium,
lithium, etc.) or ammonium.
Formula 1
Aminotri (Methylenephosphonic Acid) ##STR2## Formula 2
Aminotri (Methylenephosphonic Acid) 5Na Salt ##STR3## Formula 3
1-hydroxyethylidene-1, 1-diphosphonic acid ##STR4## Formula 4
1-hydroxyethylidene-1, 1-diphosphonic acid 3Na Salt ##STR5##
Formula 5
1-hydroxyethylidene-1, 1-diphosphonic acid 4Na Salt ##STR6##
Formula 6
Ethylenediaminetetra (Methylenephosphonic Acid) ##STR7## Formula
7
Diethylenetriaminepenta (Methylenephosphonic Acid) ##STR8##
The combination of at least two organophosphonate compounds listed
above is also useful.
The concentration of the organophosphonate compound in the aqueous
solution is preferably 0.05 mmol/l to 50 mmol/l, and more
preferably, 0.5 mmol/l to 5 mmol/l. When the concentration of the
organophosphonate compound is less than 0.05 mmol/l, corrosion
resistance and coating adhesiveness are insufficient, and chelation
ability to tin ions is inferior, resulting in production of sludge.
When exceeding 50 mmol/l, etching of the metal surface is
accelerated too much, resulting in bad appearance of a can, and in
addition, the corrosion resistance is reduced.
A supply source of the tin ions (third component) is stannous
chloride, stannic chloride, sodium stannate, stannous sulfate, and
so on. At least two of these may be mixed with divalent and
tetravalent tin ions to be used as a supply source of tin ion. The
concentration of the tin ions in the surface treating aqueous
solution is preferably 0.005 to 5.0 g/l (i.e. 0.042 to 42 mmol/l,
and more preferably, 0.05 to 0.5 g/l (i.e. 0.42 to 4.2 mmol/l).
When the concentration of the tin ions is less than 0.005 g/l, the
corrosion resistance and coating adhesiveness are inferior in the
resultant conversion coating. When exceeding 5.0 g/l, a large
amount of sludge is produced in the treating bath, and working
efficiency is reduced.
A mole ratio of tin ion (Sn) to organophosphonate compound is
preferably 0.08 to 8.4, and more preferably, 0.17 to 2.8. When the
mole ratio is smaller than 0.08, the corrosion resistance and
coating adhesiveness are inferior in the resultant conversion
coating. When the mole ratio exceeds 8.4, a large amount of sludge
is produced in the treating bath, and working efficiency is
reduced.
Even if the mole ratio is greater than 1, sludge is not always
produced. However, in order to sufficiently chelate the tin ions,
the most preferable mole ratio Sn/organophosphonate compound is
0.17 to 1.
The pH value of the surface treatment aqueous solution for metal
must be less than 5. The pH value is preferably set to 2 to 5, and
more preferably 2 to 3. When the pH value of the surface treatment
aqueous solution is less than 2, the metal surface is etched too
much, resulting in bad appearance, as well as the resultant
conversion coating having an inferior corrosion resistance. When
the pH value of the surface treatment aqueous solution is greater
than 5, chemical conversion (reaction) does not satisfactorily
advance, which makes it difficult to form a conversion coating
film.
The surface treatment aqueous solution in accordance with the
invention is prepared by diluting a thick treatment solution with
an appropriate amount of water to an appropriate concentration
range, and is poured into a treatment bath for use. The thick
treatment solution contains at least orthophosphoric acid and/or
orthophosphate with a phosphate ion amount 10 to 1,500 g/l, 0.5 to
2,500 mmol/l of organophosphonate compound, and 0.05 to 500 g/l of
tin ion, with a pH value being 4 or less.
Although, in the surface treatment bath, phosphoric acid and
organophosphate compound are easily consumed, the concentration of
these substance can be kept constant by an automatic supply system.
Tin ions continuously dissolve into the surface treatment aqueous
solution from a tin plating film formed on the metal surface during
the chemical conversion, which will form a coating film by reacting
with an acid component which is constantly supplied into the
treatment bath.
Prior to the conversion coating using a surface treatment aqueous
solution of the invention, the surface of a tinned metal (such as
tinned iron) is degreased and water washed. Then, the the surface
treatment aqueous solution is applied to the tinned metal by an
arbitrary method, such as immersing or spraying. Treating
temperature is within the range between the room temperature and
80.degree. C., preferably, 40 to 60.degree. C. The treating time is
generally 5 seconds to 2 minutes, and preferably 20 to 60 seconds.
After the application of the surface treatment aqueous solution,
the treated can is first washed with tap water, then washed with
pure water, and is finally dried.
When applying the surface treatment aqueous solution to a tinned
surface of a can, the tin plating layer on the metal surface is
etched by phosphoric acid and organophosphonate compound. The tin
ions dissolve into the surface treatment aqueous solution, which
then reacts with phosphoric acid and organophosphate compound to
form an insoluble tin phosphate. The insoluble tin phosphate is to
cover the exposed iron surface of the can as a conversion coat with
a superior corrosion resistance and coating adhesiveness.
Besides the etching ability, the organophosphonate compound has a
chelation ability for tin ions contained in the surface treatment
aqueous solution. Condensed phosphoric acid used in the prior art
solution, such as pyrophosphoric acid or tripolyphosphoric acid,
consists of plural PO.sub.4 pyramids bonded to each other bridged
by oxygen atoms. Due to this structure, it is easily hydrolyzed
when the P--O--P bonding is attacked by hydrogen ions. Especially
at high temperature and in a strong acidic range, the condensed
phosphoric acid is remarkably hydrolyzed to become phosphoric acid
having a low degree of polymerization which lacks sufficient
chelation ability for tin ions contained in the surface treatment
aqueous solution, resulting in production of sludge.
On the contrary, organophosphate compound, in which the phosphonate
group (--PO.sub.3 H.sub.2) is bonded to a carbon atom, is scarcely
subjected to hydrolysis and can stably exist over a broad pH range.
Accordingly, chelation ability for tin ions is constant, and there
is no need to supply a large amount of chelating agent during the
conversion coating treatment, unlike the case of condensed
phosphoric acid. It is considered that a phosphonate compound which
acts to chelate tin ions is taken into a coating film as a superior
ingredient. Thus, the organophosphonate compound contained in the
surface treatment aqueous solution in accordance with the invention
has multiple functions, and serves as an etchant and chelating
agent as well as a film forming ingredient. By using such an
advantageous component, both working and economical efficiencies
are improved, with superior corrosion resistance and coating
adhesiveness.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Experimentation was carried out by comparing actual examples in
accordance with the invention with other examples for comparison.
In the experimentation, (1) external appearance of resultant cans,
(2) corrosion resistance of cans prior to epoxy coating, (3)
corrosion resistance of cans after epoxy coating, and (4) coating
adhesiveness, were evaluated in the following manner.
(1) External Appearance
The external appearance of cans, which have been subjected to
conversion coating treatment and dried, was observed with the naked
eye and evaluated according to the following scale.
Excellent: Glossy
Average: medium glossiness
Poor: Poor glossiness and yellowed surface
(2) Corrosion Resistance Prior To Epoxy Coating
A number of can side wall portions, which had been subjected to
conversion coating treatment and dried, were cut out to a size of 3
cm.times.4 cm, for the test pieces. The test pieces were taped so
that a 1 cm.times.1 cm area of the inside surface was exposed,
which was then immersed into 2.8 g/l of phosphoric acid aqueous
solution (adjusted with NaOH so as to be pH 2.4) at 40.degree. C.
for 48 hours. After that, the corrosion advancing state was
observed and evaluated according to the following scale.
Excellent: corroded area is 0%
Good: corroded area is less than 5%
Average: corroded area is 5% or more and less than 20%
Inferior: corroded area is 20% or more and less than 50%
Poor: corroded area is 50% or more
(3) Corrosion Resistance After Epoxy Coating
Commercially available epoxy coating was applied to the inside
surface of cans to a thickness of 4 to 5 .mu.m, and the coated cans
were baked at 210.degree. C. Then side wall portions of the coated
cans were cut out to a size of 5 cm.times.5 cm to make test pieces.
The exposed cut edges and back were taped, and the coated surfaces
scarred in the lateral direction to a length of 3 cm until reaching
the metal surface in the depth direction. The test pieces were
immersed into 1.3 g/l of citric acid aqueous solution (adjusted by
NaOH to be pH 3.6) at 40.degree. C. for 30 days. After that, the
corrosion advancing state was observed and evaluated according to
the following scale.
Excellent: Width of corroded area is smaller than 5 mm
Good: Corroded width is 5 mm or more, and less than 10 mm
Average: Corroded width is 10 mm or more, and less than 15 mm
Inferior: Corroded width is 15 mm or more, and less than 20 mm
Poor: Corroded width is 20 mm or more
(4) Coating adhesiveness
The same epoxy coated cans as those used in the test (3) were used
in this test. Side wall portions of the coated cans were cut out to
a size of 5 cm.times.10 cm as test portions, which were immersed in
50 g/l of boiling acetic acid aqueous solution for 30 minutes. The
coated surfaces of the test pieces were scratched so as to draw a
hundred 1 mm.times.1 mm squares like a chessboard to the depth of
the metal surface. Then, an adhesive tape was stuck onto the
scratched portions and quickly peeled off. The peeling conditions
were observed and evaluated according to the following scale.
Excellent: Peeled area is 0%
Good: Peeled area is less than 5%
Average: Peeled area is 5% or more, and less than 20%
Inferior: Peeled area is 20% or more, and less than 50%
Poor: Peeled area is 50% or more
ACTUAL EXAMPLES 1 to 25 IN ACCORDANCE WITH THE INVENTION AND
COMPARISON EXAMPLES 1 to 6
Tinned steel plate with 2.8 g/m.sup.2 of tin plating was employed
to fabricate DI cans (i.e. cans made by a DI technique). The DI
cans were degreased using 10 g/l of an alkaline degreasing agent
(SURFCLEANER SN311, Nippon Paint Co., Ltd.; "SURFCLEANER" is a
registered trademark), and were water washed. Then, the cans were
subjected to spray treatment at 50.degree. C. for 30 seconds with a
surface treatment aqueous solution having a composition and a pH
value shown in Tables 1 to 4. In the experimentation,
orthophosphonic acid (H.sub.3 PO.sub.4) was used as a supply source
of "PO.sub.4.sup.3- ". As an organophosphonate compound,
1-hydroxyethylidene-1,1-diphosphonic acid (referred to as "A" in
the tables) and/or aminotri (methylenephosphonic acid) (referred to
as "B" in the tables) were used. SnCl.sub.2 2H.sub.2 O (Sn.sup.2+
in the aqueous solution) and/or Na.sub.2 SnO.sub.3 3H.sub.2 O
(Sn.sup.4+ in the aqueous solution) were added for a supply source
of tin ions. The pH value was adjusted by sodium hydroxide (NaOH).
After the spray treatment, the cans were washed first with tap
water, then with pure water, and finally dried at 200.degree. C.
for 3 minutes.
The external appearance and corrosion resistance of the thus
processed cans were evaluated prior to epoxy resin coating.
Then, a commercially available epoxy resin coating was applied onto
the DI cans, which had been subjected to the conversion coating, to
a thickness of 4 to 5 .mu.m. The coated cans were baked at
210.degree. C.
The corrosion resistance after epoxy resin coating and coating
adhesiveness was evaluated.
Also, production of sludge was observed after leaving the cans at
room temperature for a day. Evaluation was made with the following
scale.
Excellent: No sludge was produced
Average: A little sludge was produced
Poor: A significant amount of sludge was produced
TABLE 1
__________________________________________________________________________
Composition Tin ion/ Spray Organophosphnate Organophosphnate
Treatment Treatment Actual PO.sub.4.sup.3- compound(mmol/l) Tin ion
(g/l) compound Temperature Time examples (g/l) (A) (B) Sn.sup.+2
Sn.sup.4+ (mole-ratio) (.degree.C.) pH (second)
__________________________________________________________________________
1 6.0 5.0 0 0.1 0 0.17 60 3.0 20 2 10.0 5.0 0 0.1 0 0.17 60 3.0 20
3 30.0 5.0 0 0.1 0 0.17 60 3.0 20 4 35.0 5.0 0 0.1 0 0.17 60 3.0 20
5 3.0 5.0 0 0.1 0 0.17 60 3.0 20 6 1.0 5.0 0 0.1 0 0.17 60 3.0 20 7
0.5 5.0 0 0.1 0 0.17 60 3.0 20 8 6.0 0.3 0 0.1 0 2.8 60 3.0 20 9
6.0 0.3 0 0.005 0 0.14 60 3.0 20 10 6.0 0.5 0 0.005 0 0.08 60 3.0
20 11 6.0 5.0 0 0.5 0 0.84 60 3.0 20 12 6.0 25.0 0 1.0 0 0.34 60
3.0 20 13 6.0 50.0 0 5.0 0 0.84 60 3.0 20 14 6.0 60.0 0 5.0 0 0.71
60 3.0 20 15 6.0 50.0 0 6.0 0 1.02 60 3.0 20 16 6.0 0 5.0 0.1 0
0.17 60 3.0 20 17 6.0 2.5 2.5 0.1 0 0.17 60 3.0 20 18 6.0 5.0 0
0.05 0.05 0.17 60 3.0 20
__________________________________________________________________________
TABLE 2
__________________________________________________________________________
Composition Tin ion/ Spray Organophosphnate Organophosphnate
Treatment Treatment PO.sub.4.sup.3- compound(mmol/l) Tin ion (g/l)
compound Temperature Time (g/l) (A) (B) Sn.sup.+2 Sn.sup.4+
(mole-ratio) (.degree.C.) pH (second)
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Actual Examples 19 6.0 5.0 0 0 0.1 0.17 60 3.0 20 20 6.0 5.0 0 0.1
0 0.17 60 1.5 20 21 6.0 5.0 0 0.1 0 0.17 60 2.0 20 22 6.0 5.0 0 0.1
0 0.17 60 5.0 20 23 6.0 5.0 0 0.1 0 0.17 60 5.5 20 24 6.0 10.0 0
0.06 0 0.05 60 3.0 20 25 6.0 4.2 0 5 0 10 60 3.0 20 Comparions
Examples 1 6.0 5.0 0 0 0 0 60 3.0 20 2 6.0 0 0 0 0 0 60 3.0 20 3
6.0 0 0 0.1 0 -- 60 3.0 20 4 6.0 Pyrophosphate acid 0.1 0 -- 60 3.0
20 0.2 5 6.0 Oxalic acid 0.1 0 -- 60 3.0 20 0.2 6 6.0
Hypophosphorous acid 0.1 0 -- 60 3.0 20 0.2
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TABLE 3
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Actual External Corrosion resistance of cans Corrosion resistance
of cans Coating Production examples Appearance prior to epoxy
coating after epoxy coating adhesiveness of stage
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1 Excellent Good Excellent Excellent Excellent 2 Excellent Good
Excellent Excellent Excellent 3 Excellent Good Excellent Excellent
Excellent 4 Average Good Excellent Excellent Excellent 5 Excellent
Good Excellent Excellent Excellent 6 Excellent Good Excellent
Excellent Excellent 7 Excellent Average Good Excellent Excellent 8
Excellent Good Excellent Excellent Average 9 Excellent Average
Excellent Excellent Excellent 10 Excellent Good Excellent Excellent
Excellent 11 Excellent Good Excellent Excellent Excellent 12
Excellent Good Excellent Excellent Excellent 13 Excellent Good
Excellent Excellent Excellent 14 Average Good Excellent Excellent
Excellent 15 Excellent Good Excellent Excellent Excellent 16
Excellent Good Excellent Excellent Excellent 17 Excellent Good
Excellent Excellent Excellent 18 Excellent Good Excellent Excellent
Excellent
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TABLE 4
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External Corrosion resistance of cans Corrosion resistance of cans
Coating Production Appearance prior to epoxy coating after epoxy
coating adhesiveness of slage
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Actual Examples 19 Excellent Good Excellent Excellent Excellent 20
Average Good Excellent Excellent Excellent 21 Excellent Good
Excellent Excellent Excellent 22 Excellent Good Excellent Excellent
Excellent 23 Excellent Good Good Good Excellent 24 Average Good
Excellent Excellent Excellent 25 Excellent Good Excellent Excellent
Average Comparions Examples 1 Excellent Average Good Good Excellent
2 Excellent Average Average Good Poor 3 Excellent Average Good
Excellent Poor 4 Excellent Average Good Good Poor 5 Excellent Good
Excellent Excellent Poor 6 Excellent Good Good Good Poor
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In the experimentation, comparison example 4 corresponds to a
conversion coating solution disclosed in U.S. Pat. No. 4,927,472,
comparison example 5 corresponds to a conversion coating solution
described in Japan Kokai H2-15178, and comparison example 6
corresponds to a treatment solution with inorganic hypophosphorous
acid employed in Japan Kokai 52-53739.
In the actual examples 1 to 25 using a surface treatment aqueous
solution in accordance with the present invention, production of
salt or precipitation (i.e. sludge) were scarcely seen, and the
lifetime of the conversion coating (surface treatment) aqueous
solution was substantially long. Consequently, the subsequent
transporting and printing processes were smoothly carried out,
without obstacles caused by salt or precipitations stuck onto the
surface of the cans. Furthermore, as can be seen from the tables,
external appearance, corrosion resistance and coating adhesiveness
all indicated good results.
The following are preferable modifications of the present
invention.
1. The organophosphonate compound contained in the surface
treatment aqueous solution is a compound of at least one selected
from aminotri (methylenephosphonic acid),
1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra
(methylenephosphonic acid), dimethylenetriaminepenta
(methylenephosphonic acid, and their salts.
2. The organophosphonate compound contained in the surface
treatment aqueous solution is a compound of at least one selected
from aminotri (methylenephosphonic acid),
1-hydroxyethylidene-1,1-diphosphonic acid, and their salts.
3. A supply source of the phosphate ions contained in the surface
treatment aqueous solution is at least one selected from
orthophosphoric acid and orthophosphate.
4. A supply source of tin ion contained in the surface treatment
aqueous solution is at least one selected from stannous chloride,
stannic chloride, sodium stannate, and stannous sulfate.
5. A mole ratio of tin ion to organophosphonate compound in the
surface treatment aqueous solution is 0.08 to 8.4.
6. A mole ratio of tin ion to organophosphonate compound in the
surface treatment aqueous solution is 0.17 to 2.8.
7. A surface treatment aqueous solution contains phosphoric acid
and phosphate with a phosphate ion amount of 1 to 30 g/l, 0.05 to
50.0 mmol/l of organophosphonate compound, and 0.005 to 5.0 g/l of
tin ions, with a mole ratio of tin ions to organophosphonate
compound being 0.08 to 8.4 and a pH value being 2 to 5.
8. The amount of phosphoric acid and phosphate contained in the
surface treatment aqueous solution is such that the phosphate ion
amount is 3 to 10 g/l.
9. The amount of organophosphonate compound contained in the
surface treatment aqueous solution is 0.5 mmol/l to 5.0 mmol/l.
10. The amount of tin ions contained in the surface treatment
aqueous solution is 0.05 to 0.5 g/l.
11. A surface treatment aqueous solution contains 3 to 10 g/l of
phosphate ions (in the ion reduction amount), 0.5 to 5 mmol/l of
organophosphonate compound, and 0.05 to 0.5 g/l of tin ions, with a
mole ratio of tin ions to organophosphonate compound being 0.17 to
2.8 and a pH value being 2 to 5.
12. The pH value of the surface treatment aqueous solution is 2 to
3.5.
13. A surface treatment aqueous solution contains (a) at least one
selected from orthophosphoric acid and orthophosphate with a
phosphate ion amount of 3 to 10 g/l, (b) 0.5 to 5 mmol/l of at
least one organophosphonate compound selected from aminotri
(methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic
acid, and their salt, and (c) 0.05 to 0.5 g/l of tin ions, with a
mole ratio of tin ions to organophosphonate compound being 0.17 to
1 and a pH value being 2 to 5.
14. The surface treatment aqueous solution is applied to a surface
of tinned iron.
15. The surface treatment aqueous solution is applied to a surface
of tinned iron which has been degreased and water washed.
16. The surface treatment aqueous solution is applied to a tinned
DI can.
17. A thick solution for surface treatment contains at least (a)
orthophosphoric acid or orthophosphate with a phosphate ion amount
of 10 to 1,500 g/l, (b) 0.5 to 2,500 mmol/l of organophosphonate
compound, and (c) 0.05 to 500 g/l of tin ions, with a pH value
being 4 or less.
18. A surface treatment aqueous solution is obtained by diluting
the thick solution for surface treatment with an appropriate amount
of water, and is poured into a treatment bath for use in conversion
coating.
As has been described, by using a surface treatment aqueous
solution in accordance with the present invention, conversion
coating having a superior corrosion resistance can be achieved. The
conversion coating also has a superior coating adhesiveness to an
upper coating.
During the processing, salt or precipitate is seldom produced in
the treatment bath containing the surface treatment aqueous
solution of the invention, thereby maintaining a long lifetime of
the surface treatment bath. Furthermore, later transporting and
printing processes are carried out smoothly without obstacles
caused by salt stuck onto the surface of cans.
Organophosphonate compound in which a phosphonate group (--PO.sub.3
H.sub.2) is bonded to a carbon atom can stably exist over a broad
pH range, without being subjected to hydrolysis. Accordingly, its
chelating ability for tin ions is also stable, and it is not
necessary to continuously add a large amount of chelating agent
into the treatment bath during the processing, unlike a treatment
solution containing condensed phosphoric acid. organophosphonate
compound which is acts to chelate tin ions is considered to be
taken into a conversion coating film as a preferable component. In
other words, the organophosphonate compound contained in the
surface treatment aqueous solution serves as a multifunctional
component (functioning as an etchant, a chelating agent, and an
ingredient of a conversion coating film), which can remarkably
improve both working efficiency and cost efficiency of the
treatment solution.
The thus prepared surface treatment aqueous solution is suitable
for conversion coating of the tinned surface of iron products,
especially, of tinned DI cans.
* * * * *